Suppose Apple products such as iOS devices and OS X devices at hand. You want to make things searchable. Some files such as converted PDFs lack OCR so you cannot search them. Some formats are not supported by OCR tools. Root question broken:

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I would convert them first into PDF and then use some well-known OCR-tool such as Adobe Acrobat PRO. You can get it with the Creative Suite to make documents searchable. In order to convert certain type into PDF, I would use Online Converters such as the DjVU Online Converter such as here. The OCR requires a lot of computation power so probably best done with Desktop tools unless cloud-tool or iOS tool found.